In health care, decision makers are faced with increasing innovation and demand for services accompanied by escalating costs. As a result, governments and institutions have sought to promote health care value (i.e. better outcomes per moneys spent). A summary measure of health-related quality-of-life (HRQoL) to help decide how to allocate available resources is thus highly desirable. In no other area of public policy has a measure similar to the widely-used quality-adjusted life-year (QALY) been developed. The QALY is therefore unique in both its ambitions and in the political, philosophical and measurement challenges it faces. This thesis set out to examine health state valuation using the time-trade off (TTO), a tool used to measure HRQoL, in the context of a behavioural economic framework. Observed violations of procedural and descriptive invariance, cornerstones of decision theory (on which the TTO is based), have been witnessed in health state valuation and elsewhere. Behavioural economics offers a framework by which such inconsistencies can potentially be better understood. Although behavioural economics has gained traction in other areas of decision research, its application to health state valuation has been limited. Drawing on the decision-making literature and health-specific considerations, the empirical studies in this thesis: provide insight into why previous studies of the TTO have yielded inconsistent findings, showcase violations of internal consistency due to behavioural economic phenomena, and identify issues relevant to the choice of TTO ‘version’ (i.e. how values should be elicited). Implications of the research in terms of stated preference methods and their role in policy are discussed. A strict focus on the TTO was intended, as it is the tool most widely implemented in health state preference elicitation, both in research contexts and clinical studies that seek to demonstrate cost-effectiveness. However, importantly, the empirical findings and discussion in this thesis are relevant not only to researchers of health state valuation but to policy makers in health and other areas of social policy which seek input for their decisions through stated preference exercises.